done essentially to obtain the proper distance betweenthe lens and the film. When light rays come from a farobject and pass through a lens, they form a sharp imageclose to the lens. When light rays come from a nearobject, they form an image farther away from the lens.This means that the lens must be focused on either the

basically an accumulation of many points that are exactimages of points composing the subject. After lightstrikes a subject, it is reflected from many points on thesubject. A camera lens redirects these reflected rays intocorresponding points on the film. Each of these pointsis reproduced by the lens as a circle. When the circle issmaller than l/100 inch, it appears as a sharp point tothe eye. When the circle is larger than 1/100 inch, theeye sees it as a circle, and the image is blurred or out offocus. Each out-of-focus circle on the film is called acircle of confusion and can be visualized as the crosssection of a cone of a light ray (fig. 1-27).